Vegetto is formed when Goku and Vegita each wear one of the Fusion earrings. The earrings are designed so that when anyone is fused, they will be fused forever instead of the original 30 minutes. Vegetto arrives on the scene in the fight against Majin Buu. During this time, Buu has become more powerful than ever, already aborsbing the likes of piccilo, gohan, and gotenks. Vegitto is very powerful and has the highest power level of any of the fused saiyans.
Vegetto could easily kick the crap out of Buu, as it was shown throught out the fights they had. But Vegitto's original plans were a bit different. He went through one of the pores of Buu to retrieve his friends that were already absorbed. While inside Buu, he was able to rescue all of his friends including the fat good buu. Upon doing so he tunred Buu back into his original form and he also became unfused.